The present invention is adapted for use with a conventional drive system in which a driven sprocket and a drive sprocket are connected by a flexible drive element such as a drive belt. In such an arrangement, it is well known that the proper functioning of the drive system and the operating life of the drive belt can be significantly enhanced by maintaining the two drive belt runs between the sprockets in tension such that significant slack does not occur in either run. One method of providing such tension is to resiliently bias the sprockets away from one another. However, a more convenient arrangement is provided by a tensioning device that biases one or both drive belt runs inwardly towards the other run at a point intermediate the sprockets. This arrangement provides the added advantage of keeping a greater surface area of the drive belt in contact with the outer surface of the sprockets at any given period of time. A number of examples of this latter type of tensioning device are disclosed in the prior art. However, in the great majority of prior tensioning devices, the tensioning device is adapted to operate in a particular drive system, e.g., to operate with a given arrangement of sprockets and drive belt.
One result of the fact that prior tensioning devices have been adapted for specific applications is that most such devices have been mounted or secured to a support that is fixed with respect to the sprocket axles. Through use of such a support arrangement, the position of the tensioning device between the sprockets cannot be controlled, to maintain the tensioning device at the optimum position. The use of fixed supports has been viewed as especially important for tensioning devices adapted to operate with sprockets that are or may be horizontally positioned with respect to one another. In such a horizontal arrangement, the weight of the tensioning device does not affect its position, i.e., the position of the tensioning device between the sprockets is not in any way controlled by gravity acting on the tensioning device.
A further feature of the majority of prior art tensioning devices is that in such devices, the surfaces contacting the two runs of the drive belt and urging them inward are either fixed in position with respect to one another, or resiliently biased towards one another by springs or similar means. A disadvantage of the resilient biasing technique is that it adds complexity to the tensioning device, and the spring or other resilient means are themselves subject to wear over time. Tensioning devices having fixed distances between their contact surfaces cannot be adjusted as the belt experiences increased wear. Thus, the tensioning force provided by the device decreases over time, at least in those arrangements where the sprockets are horizontally positioned with respect to one another. A further disadvantage of a fixed distance between the contact surfaces is that the tensioning device cannot be used with different sized sprockets.
One attempt to overcome the drawbacks of the prior art has been provided by the belt tensioner disclosed in U.S. Pat. No. 4,068,535 to Sheets. This device utilizes upper and lower idler assemblies, each of which carries significantly longitudinally spaced rollers, which are adjustably connected by threaded rods. However, this device is not easily adjusted, due to the need to undo locking washers and nuts, and is not compact, due to the widely spaced rollers. Thus, this device is not well suited for use on drive belts which operate in closely confined spaces. Further, the rollers included in this device are mounted on axles, which increases the complexity of assembly and may be subject to wear.
Additional partial solutions for tensioning drive mechanism tensioners are provided by the chain tensioners disclosed in U.S. Pat. Nos. 4,662,862 to Matson, 4,798,562 to Matson et al. and 4,850,934 to Gibson Jr. et al. These devices provide upper and lower low friction drive chain contact members that glide on chain surfaces, and which are incremented by adjustable straps. The straps are readily adjusted to change the spacing of the contact members, particularly in a disclosed ratcheting strap version. However, these devices, while well suited for use on chains, are not as well suited for use on belts due to the increased friction associated with belt contact on the contact members.